This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-052062, filed Feb. 27, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube having a press-molded shadow mask.
2. Description of the Related Art
In general, an in-line color cathode ray tube comprises a vacuum envelope, which includes a glass panel having a substantially rectangular effective portion, a glass funnel connected to the panel, and a cylindrical glass neck connected to a small-diameter portion of the funnel. Formed on the inner surface of the effective portion of the panel is a phosphor screen that includes three-color dotted or striped phosphor layers, which glow blue, green, and red, individually, and black light-shielding layers. In the vacuum envelope, a shadow mask having a large number of electron beam passage apertures is opposed to the phosphor screen. Further, an in-line electron gun that emits three electron beams is located in the neck, and a deflection yoke is mounted on the outer peripheral surface of the funnel.
In the color cathode ray tube constructed in this manner, the three electron beams emitted from the electron gun are deflected horizontally and vertically by means of horizontal and vertical deflecting magnetic fields that are generated by the deflection yoke. The phosphor screen is horizontally scanned at high frequency with the electron beams that are passed through the shadow mask, whereupon a color image is displayed. As this is done, the electron beams emitted from the electron gun land on the phosphor screen at given angles of incidence, and undergo color selection on the shadow mask according to the angles of incidence. Thus, the three electron beams correspond individually to the three colors, red, blue, and green, of the phosphor screen.
In this color cathode ray tube, the shadow mask includes a substantially rectangular effective portion in the form of a gently sloped dome, and a skirt portion extending substantially perpendicularly from the peripheral edge of the effective portion toward the electron gun. The effective portion has a curvature radius of about 500 to 2,000 mm and a large number of electron beam passage apertures with a diameter of about 100 xcexcm. The shadow mask is fixed to a mask frame by means of the skirt portion. The mask frame is detachably supported on stud pins on the panel by means of elastic support members.
Thus, the shadow mask carries out the aforesaid color selection and serves as a negative of a phosphor screen in the process of manufacturing the phosphor screen. Usually, the shadow mask is formed into a desired shape by press-molding a thin sheet of Invar (iron-nickel alloy) with a thickness of 0.1 to 0.2 mm.
Since the Invar material is springy, however, it is hard to obtain a desired mask surface shape from it. In forming the shadow mask by press molding, a flat mask blank is first placed on a knockout and a die of a pressing machine. Then, the mask blank is fixed by holding a fixing portion in the peripheral part of the blank by means of a blank holder and the die. After the mask blank is bulged into a specific curved surface by means of a punch, the blank holder and the die are separated from each other to release the peripheral part of the blank.
Then, the knockout and the punch are moved downward, and the peripheral part of the mask blank is drawn into a space between the punch and the die to be bent substantially at right angles, whereupon the skirt portion is formed. Thereafter, the punch is pulled up, all the tools are restored to their respective original positions, and the molded shadow mask is taken out.
In the case of the shadow mask that is obtained by press molding, however, the effective portion is molded in the form of a gently sloped dome, so that the boundary between the effective portion and the skirt portion and an extending end edge of the skirt portion are arcuate, and their respective lengths are shorter than before press molding operation. Thus, a greater odd of the blank is produced in a part of the skirt portion that is situated closer to the extending end, so that wrinkles are formed in the skirt portion.
Although these wrinkles can be reduced to some degree by notching the extending end edge of the skirt portion, deep notches are needed to remove the wrinkles thoroughly. If the notches are deep, however, they divide the skirt portion so sharply that the shape retention of the shadow mask lowers extremely. Therefore, the shadow mask is inevitably deformed with ease while it is being transported or attached to the mask frame.
If the notches are shallow, a lot of wrinkles are inevitably formed in the skirt portion, and compressive stress that is attributable to the odd of the blank remains in the unnotched region. Force that urges the skirt portion to spread acts on the skirt corner portions where no drag acts against the compressive stress. Since this force pushes back the arcuate skirt portion straight, it acts on the effective portion as an urging force to displace each corner portion of the boundary between the effective portion and the skirt portion upward (or toward the phosphor screen), and the central portion of each side downward (or toward the electron gun).
The stiffness of the central portion of each side of the mask effective portion against the aforesaid downward force is so low that downward displacement is caused in the central portion. In the case of a 17-inch in-line color cathode ray tube, for example, depressions of 0.2 mm to 0.3 mm are formed inevitably. Accordingly, the curved surface of the shadow mask varies from a designed one, so that the electron beams are subject to landing errors, and the color purity lowers. Further, the depressions in the effective portion produce inflection points in the mask effective portion, so that the tensile strength of the mask effective portion lowers. Thus, the shadow mask is inevitably deformed with ease while it is being transported or attached to the mask frame.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube with improved display characteristics, in which an odd of a skirt portion that is produced as a shadow mask is molded can be absorbed to reduce errors in the curved surface shape of the shadow mask.
In order to achieve the above object, a color cathode ray tube according to an aspect of the invention comprises: a panel having a phosphor screen on the inner surface thereof, the phosphor screen having a plurality of phosphor layers; an electron gun located opposite the phosphor screen and configured to emit electron beams toward the phosphor screen; and a shadow mask located opposite the phosphor screen and having a large number of electron beam passage apertures through which the electron beams are applied to the phosphor layers corresponding thereto, the shadow mask being formed by press molding and including a substantially rectangular mask effective portion in the form of a gently sloped dome having the electron beam passage apertures, and a skirt portion extending from the peripheral edge of the mask effective portion substantially at right angles thereto, the skirt portion having a plurality of apertures arranged to be spaced from one another in a direction parallel to the peripheral edge of the mask effective portion and belt portions defined between the apertures and an extending end edge of the skirt portion and extending along the extending end edge, the belt portions having wrinkles formed along the extending end edge by the press molding.
A color cathode ray tube according to another aspect of the invention comprises: a panel having a phosphor screen on the inner surface thereof, the phosphor screen having a plurality of phosphor layers; an electron gun located opposite the phosphor screen and configured to emit electron beams toward the phosphor screen; and a shadow mask located opposite the phosphor screen and having a large number of electron beam passage apertures through which the electron beams are applied to the phosphor layers corresponding thereto, the shadow mask being formed by press molding and including a substantially rectangular mask effective portion in the form of a gently sloped dome having the electron beam passage apertures, and a skirt portion extending from the peripheral edge of the mask effective portion substantially at right angles thereto, the skirt portion having a plurality of slit groups arranged to be spaced from one another in a direction parallel to the peripheral edge of the mask effective portion and belt portions defined between the slit groups and an extending end edge of the skirt portion and extending along the extending end edge, each of the slit groups including a plurality of slits extending substantially at right angles to the extending end edge of the skirt portion and arranged at spaces in a direction substantially parallel to the extending end edge, the slits including a central slit, the longest one, and side slits arranged on the opposite sides of the central slit and having lengths reduced stepwise.
According to the color cathode ray tube constructed in this manner, the apertures or slit groups can absorb an odd of the skirt portion that is produced as the shadow mask is molded, thereby preventing the skirt portion from being spread by the odd. Thus, the resulting color cathode ray tube enjoys higher accuracy for the curved surface of the mask effective portion and improved display characteristics. At the same time, the belt portions can maintain the strength of the short side portion and improve the resistance to impact.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.